The present invention is related to and apparatus and method for generating low currents. More particularly, the present invention is related to current generators that accurately generate low currents using switched capacitor techniques.
An accurate current can be generated with an operational amplifier that is arranged in a feedback loop. The operational amplifier (op-amp) is arranged to force a known reference voltage across a known resistor value. The resistor and op-amp are arranged to convert the reference voltage to current. An example current generator circuit (400) that uses an operational amplifier is illustrated in FIG. 4.
As shown in FIG. 4, current generator circuit (400) includes an N-type field effect transistor (FET MSF), three resistors (R41-R43), and an operational amplifier (AMP40). FET MSF includes a gate that is connected to a control node, a drain that is connected to an output node (OUT), and a source that is connected to a feedback node. Resistor R41 is connected between the feedback node and ground. Resistors R42 and R43 are series connected between a voltage reference terminal (VREF) and ground. Amplifier AMP40 includes a non-inverting input that is connected to a common node between resistors R42 and R43, an inverting input that is connected to the feedback node, and an output that is coupled to the control node.
In operation, a reference voltage (VREF) is applied to the current generator circuit (400). Resistors R42 and R43 operate as a resistor divider that provides a second reference voltage (VR2) in response to the reference voltage (VREF). FET MSF receives a control voltage (VCTL) from the output of amplifier AMP40 and produces an output current (IOUT). The output current (IOUT) flows through resistor R41, which produces a feedback voltage (VFB). Amplifier AMP40 provides the control voltage (VCTL) in response to a comparison between the second reference voltage (VR2) and the feedback voltage (VFB). Amplifier AMP40 provides control of FET MSF such that output current IOUT is determined by the second reference voltage (VR2) divided by the resistance of R41.
The present invention is directed to a method and apparatus for accurately generating low currents using switched capacitor techniques. The current generator includes a reference voltage generator that provides a reference signal to a switched capacitor integrator. In one example, the reference circuit includes a switched capacitor divider. The switched capacitor integrator circuit produces a voltage ramp in response to the reference signal and other timing signals. The rate of the voltage ramp is proportional to the ratio of capacitors in the switched capacitor integrator and a clock frequency that is associated with the timing signals. A feedback circuit impresses the voltage ramp across an output capacitor circuit that has a very low capacitance value. The capacitor is arranged to differentiate the voltage ramp to produce an accurate low current. The switched capacitor design is suitable for integration in a monolithic integrated circuit. The integrator and the feedback stage are periodically reset.